Hello World for Dropbox apps?

I want a very simple testbed for Dropbox apps written in straight JavaScript. Nothing extra. Just: 1. Get authorized. 2. Write a file. 3. Nothing more!

Using exactly the best practices that Dropbox wants us to use. I should be able to have something running in ten minutes, or less. Nothing but JavaScript and HTML. No pre-processors. No complicated builds. Let's drop the learning curve.